Subgroup: Expression

Class: QgsExpression

class qgis.core.QgsExpression(expr: str)

Bases: sip.wrapper

Creates a new expression based on the provided string. The string will immediately be parsed. For optimization prepare() should always be called before every loop in which this expression is used.

QgsExpression(other: QgsExpression) Create a copy of this expression. This is preferred over recreating an expression from a string since it does not need to be re-parsed.

QgsExpression() Create an empty expression.

New in version 3.0.

Class for parsing and evaluation of expressions (formerly called “search strings”). The expressions try to follow both syntax and semantics of SQL expressions.

Usage:

Similarly to SQL, this class supports three-value logic: true/false/unknown. Unknown value may be a result of operations with missing data (NULL). Please note that NULL is different value than zero or an empty string. For example 3 > NULL returns unknown.

There is no special (three-value) ‘boolean’ type: true/false is represented as 1/0 integer, unknown value is represented the same way as NULL values: NULL QVariant.

For better performance with many evaluations you may first call prepare(fields) function to find out indices of columns and then repeatedly call evaluate(feature).

Operators and functions that expect arguments to be of a particular type automatically convert the arguments to that type, e.g. sin(‘2.1’) will convert the argument to a double, length(123) will first convert the number to a string. Explicit conversion can be achieved with to_int, to_real, to_string functions. If implicit or explicit conversion is invalid, the evaluation returns an error. Comparison operators do numeric comparison in case both operators are numeric (int/double) or they can be converted to numeric types.

This class is implicitly shared, copying has a very low overhead. It is normally preferable to call QgsExpression( otherExpression ) instead of QgsExpression( otherExpression.expression() ). A deep copy will only be made when prepare() is called. For usage this means mainly, that you should normally keep an unprepared master copy of a QgsExpression and whenever using it with a particular QgsFeatureIterator copy it just before and prepare it using the same context as the iterator.

Implicit sharing was added in 2.14

Methods

BuiltinFunctions
Functions
areaUnits Returns the desired areal units for calculations involving geomCalculator(), e.
checkExpression Tests whether a string is a valid expression.
cleanRegisteredFunctions Deletes all registered functions whose ownership have been transferred to the expression engine.
createFieldEqualityExpression Create an expression allowing to evaluate if a field is equal to a value.
distanceUnits Returns the desired distance units for calculations involving geomCalculator(), e.
dump Return an expression string, constructed from the internal abstract syntax tree.
evalErrorString Returns evaluation error
evaluate Evaluate the feature and return the result.
evaluateToDouble Attempts to evaluate a text string as an expression to a resultant double value.
expression Return the original, unmodified expression string.
formatPreviewString Formats an expression result for friendly display to the user.
formatVariableHelp Returns formatted help text for a variable.
functionCount Returns the number of functions defined in the parser
functionIndex return index of the function in Functions array
geomCalculator Return calculator used for distance and area calculations (used by $length, $area and $perimeter functions only)
group Returns the translated name for a function group.
hasEvalError Returns true if an error occurred when evaluating last input
hasParserError Returns true if an error occurred when parsing the input expression
helpText Returns the help text for a specified function.
isField Checks whether an expression consists only of a single field reference
isFunctionName tells whether the identifier is a name of existing function
isValid Checks if this expression is valid.
needsGeometry Returns true if the expression uses feature geometry for some computation
parserErrorString Returns parser error
prepare Get the expression ready for evaluation - find out column indexes.
quotedColumnRef Returns a quoted column reference (in double quotes)
quotedString Returns a quoted version of a string (in single quotes)
quotedValue Returns a string representation of a literal value, including appropriate quotations where required.
referencedAttributeIndexes Return a list of field name indexes obtained from the provided fields.
referencedColumns Get list of columns referenced by the expression.
referencedVariables Return a list of all variables which are used in this expression.
registerFunction Registers a function to the expression engine.
replaceExpressionText This function replaces each expression between [% and %] in the string with the result of its evaluation with the specified context
rootNode Returns root node of the expression.
setAreaUnits Sets the desired areal units for calculations involving geomCalculator(), e.
setDistanceUnits Sets the desired distance units for calculations involving geomCalculator(), e.
setEvalErrorString Set evaluation error (used internally by evaluation functions)
setExpression Set the expression string, will reset the whole internal structure.
setGeomCalculator Sets the geometry calculator used for distance and area calculations in expressions.
unregisterFunction Unregisters a function from the expression engine.
variableHelpText Returns the help text for a specified variable.

Signals

Attributes

soBbox
soContains
soCrosses
soDisjoint
soEquals
soIntersects
soOverlaps
soTouches
soWithin
BuiltinFunctions() → List[str]
Functions() → object
class SpatialOperator

Bases: int

areaUnits(self) → QgsUnitTypes.AreaUnit

Returns the desired areal units for calculations involving geomCalculator(), e.g., “$area”.

Note

areas are only converted when a geomCalculator() has been set

New in version 2.14.

See also

setAreaUnits()

See also

distanceUnits()

checkExpression(text: str, context: QgsExpressionContext) → Tuple[bool, str]

Tests whether a string is a valid expression.

Parameters:
  • text – string to test
  • context – optional expression context
  • errorMessage – will be filled with any error message from the validation
Returns:

true if string is a valid expression

New in version 2.12.

cleanRegisteredFunctions()

Deletes all registered functions whose ownership have been transferred to the expression engine.

New in version 2.12.

createFieldEqualityExpression(fieldName: str, value: Any) → str

Create an expression allowing to evaluate if a field is equal to a value. The value may be null.

Parameters:
  • fieldName – the name of the field
  • value – the value of the field
Returns:

the expression to evaluate field equality

New in version 3.0.

distanceUnits(self) → QgsUnitTypes.DistanceUnit

Returns the desired distance units for calculations involving geomCalculator(), e.g., “$length” and “$perimeter”.

Note

distances are only converted when a geomCalculator() has been set

New in version 2.14.

See also

areaUnits()

dump(self) → str

Return an expression string, constructed from the internal abstract syntax tree. This does not contain any nice whitespace formatting or comments. In general it is preferable to use expression() instead.

evalErrorString(self) → str

Returns evaluation error

evaluate(self) → Any

Evaluate the feature and return the result.

Note

this method does not expect that prepare() has been called on this instance

New in version 2.12.

evaluate(self, context: QgsExpressionContext) -> Any Evaluate the expression against the specified context and return the result.

Parameters:context – context for evaluating expression

Note

prepare() should be called before calling this method.

New in version 2.12.

evaluateToDouble(text: str, fallbackValue: float) → float

Attempts to evaluate a text string as an expression to a resultant double value.

Parameters:
  • text – text to evaluate as expression
  • fallbackValue – value to return if text can not be evaluated as a double
Returns:

evaluated double value, or fallback value

New in version 2.7.

Note

this method is inefficient for bulk evaluation of expressions, it is intended for one-off evaluations only.

expression(self) → str

Return the original, unmodified expression string. If there was none supplied because it was constructed by sole API calls, dump() will be used to create one instead.

formatPreviewString(value: Any) → str

Formats an expression result for friendly display to the user. Truncates the result to a sensible length, and presents text representations of non numeric/text types (e.g., geometries and features).

Parameters:value – expression result to format
Returns:formatted string, may contain HTML formatting characters

New in version 2.14.

formatVariableHelp(description: str, showValue: bool = True, value: Any = None) → str

Returns formatted help text for a variable.

Parameters:
  • description – translated description of variable
  • showValue – set to true to include current value of variable in help text
  • value – current value of variable to show in help text

See also

helpText()

New in version 3.0.

functionCount() → int

Returns the number of functions defined in the parser

Returns:The number of function defined in the parser.
functionIndex(name: str) → int

return index of the function in Functions array

geomCalculator(self) → QgsDistanceArea

Return calculator used for distance and area calculations (used by $length, $area and $perimeter functions only)

See also

distanceUnits()

See also

areaUnits()

group(group: str) → str

Returns the translated name for a function group.

Parameters:group – untranslated group name
hasEvalError(self) → bool

Returns true if an error occurred when evaluating last input

hasParserError(self) → bool

Returns true if an error occurred when parsing the input expression

helpText(name: str) → str

Returns the help text for a specified function.

Parameters:name – function name
isField(self) → bool

Checks whether an expression consists only of a single field reference

New in version 2.9.

isFunctionName(name: str) → bool

tells whether the identifier is a name of existing function

isValid(self) → bool

Checks if this expression is valid. A valid expression could be parsed but does not necessarily evaluate properly.

New in version 3.0.

needsGeometry(self) → bool

Returns true if the expression uses feature geometry for some computation

parserErrorString(self) → str

Returns parser error

prepare(self, context: QgsExpressionContext) → bool

Get the expression ready for evaluation - find out column indexes.

Parameters:context – context for preparing expression

New in version 2.12.

quotedColumnRef(name: str) → str

Returns a quoted column reference (in double quotes)

See also

quotedString()

See also

quotedValue()

quotedString(text: str) → str

Returns a quoted version of a string (in single quotes)

See also

quotedValue()

quotedValue(value: Any) → str

Returns a string representation of a literal value, including appropriate quotations where required.

Parameters:value – value to convert to a string representation

New in version 2.14.

See also

quotedString()

quotedValue(value: Any, type: QVariant.Type) -> str Returns a string representation of a literal value, including appropriate quotations where required.

Parameters:
  • value – value to convert to a string representation
  • type – value type

New in version 2.14.

See also

quotedString()

referencedAttributeIndexes(self, fields: QgsFields) → object

Return a list of field name indexes obtained from the provided fields.

New in version 3.0.

referencedColumns(self) → Set[str]

Get list of columns referenced by the expression.

Note

If the returned list contains the QgsFeatureRequest.AllAttributes constant then all attributes from the layer are required for evaluation of the expression. QgsFeatureRequest.setSubsetOfAttributes automatically handles this case.

referencedVariables(self) → Set[str]

Return a list of all variables which are used in this expression. If the list contains a NULL QString, there is a variable name used which is determined at runtime.

New in version 3.0.

registerFunction(function: QgsExpressionFunction, transferOwnership: bool = False) → bool

Registers a function to the expression engine. This is required to allow expressions to utilize the function.

Parameters:
  • function – function to register
  • transferOwnership – set to true to transfer ownership of function to expression engine
Returns:

true on successful registration

replaceExpressionText(action: str, context: QgsExpressionContext, distanceArea: QgsDistanceArea = None) → str

This function replaces each expression between [% and %] in the string with the result of its evaluation with the specified context

Additional substitutions can be passed through the substitutionMap parameter

Parameters:
  • action – The source string in which placeholders should be replaced.
  • context – Expression context
  • distanceArea – Optional QgsDistanceArea. If specified, the QgsDistanceArea is used for distance

and area conversion

New in version 2.12.

rootNode(self) → QgsExpressionNode

Returns root node of the expression. Root node is null is parsing has failed

setAreaUnits(self, unit: QgsUnitTypes.AreaUnit)

Sets the desired areal units for calculations involving geomCalculator(), e.g., “$area”.

Note

areas are only converted when a geomCalculator() has been set

New in version 2.14.

See also

areaUnits()

setDistanceUnits(self, unit: QgsUnitTypes.DistanceUnit)

Sets the desired distance units for calculations involving geomCalculator(), e.g., “$length” and “$perimeter”.

Note

distances are only converted when a geomCalculator() has been set

New in version 2.14.

See also

distanceUnits()

See also

setAreaUnits()

setEvalErrorString(self, str: str)

Set evaluation error (used internally by evaluation functions)

setExpression(self, expression: str)

Set the expression string, will reset the whole internal structure.

New in version 3.0.

setGeomCalculator(self, calc: QgsDistanceArea)

Sets the geometry calculator used for distance and area calculations in expressions. (used by $length, $area and $perimeter functions only). By default, no geometry calculator is set and all distance and area calculations are performed using simple Cartesian methods (ie no ellipsoidal calculations).

Parameters:calc – geometry calculator. Ownership is not transferred. Set to a None to force

Cartesian calculations.

See also

geomCalculator()

soBbox = 0
soContains = 2
soCrosses = 3
soDisjoint = 5
soEquals = 4
soIntersects = 1
soOverlaps = 6
soTouches = 7
soWithin = 8
unregisterFunction(name: str) → bool

Unregisters a function from the expression engine. The function will no longer be usable in expressions.

Parameters:name – function name
variableHelpText(variableName: str) → str

Returns the help text for a specified variable.

Parameters:variableName – name of variable

See also

helpText()

New in version 2.12.